Novel Building Blocks for the Synthesis of Organic Metals.

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The structure of 7 is characterized by two parallel Cp*
rings; the distance between the two Co atoms (2.253(1) A) is
0.08 A shorter than in [Cp:Co, (p-CO),].t'51This finding
and the effective atomic number (EAN) rule indicate the
presence of a nonbridged M-M double bond, for which, on
the whole, only one example has been established." 6 ] The
relationship to 4 merely involves the addition of two CO
bridges. These conclusions are contradicted, however, by the
'H NMR spectrum of 7 (6 = 61.3), which supports a marked
paramagnetic component. Clearly, theoretical chemists have
much to look into here! Similarly, a certain resistance of 7
toward C,H, and Co (at least under the normal conditions
used so far) seems not to support the presence of a Co-Co
double bond.
The question now is whether relationships to the reactivity
of 4 , the pentamethylated derivative of 4, can be established.
A salient feature of ligand-bridged multiple-bond complexes
I
I
Se-Se
is that, usually, they readily undergo addition reactions involving opening of the bridge and only afterwards do they
undergo substitution reactions [Eq. (a)] ; the analogous
transformation could occur directly from 7, starting with addition.'"I
The prospect of being able to investigate the bonding and
reactivity of nonbridged organometallic multiple-bond systems is why the successful synthesis of 7 is so significant. A
sure goal will be to extend the preparation to other transition
metals.
German version: Angew. Chem. 103 (1991) 1140
[l] E. Peligot, C. R . Hebd. Seances Acad. Sci. 19(1844)609; Ann. Chim. P h w
f 2 (1844) 528.
[2] E A. Cotton, B. G. DeBoer, M. D. La Prada, J. R. Pipal, D. A. Ucko,
Acta Crystallogr. Sect. B27 (1971) 1664.
131 F. A. Cotton, T. E. Haas, Inorg. Chem. 3 (1964) 10.
[4] F. A. Cotton, N. F. Curtis, C. B. Harris, B. E G. Johnson, S. J. Lippard,
J. T. Mague, W. R. Robinson, J. S. Wook, Science 145 (1964) 1305.
[5] F. A. Cotton, R. A. Walton: Multiple Bonds Between MetalAtoms, Wiley,
New York 1982, S. 4.
161 I. Bernal, J. D. Korp, G. M. Reisner, W. A. Herrmann, J. Organomer.
Chem. 139 (1977) 321.
171 A review on compounds of this type is given in [5], p. 245f.
[8] J.-S. Huang, L. F. Dahl, J. Organomet. Chem. 243 (1983) 57; E. D. Jemmis,
A. R. Pinhas, R. Hoffmann, J. Am. Chem. Soc. 102 (1980) 2576.
191 J. J. Schneider, R. Goddard, S. Werner, C. Kriiger, Angew. Chem. 103
(1991) 1145; Angew. Chem. Int. Ed. Engl. 30 (1991) 1124.
[lo] M. P. Andrews, G. A. Ozin, J. Phys. Chem. 90 (1986) 1245.
[ I l l P. L. Timms, Chem. Commun. 1969, 1033.
[I21 J. R. Blackborow, D. Young: Metal Vupour Synthesis in Organomerallic
Chemistry, Springer, Berlin 1979, p. 12Of.
[13] G. N. Cloke, J. P. Day, J. C. Green, C. P. Morley, A. C. Swain. J. Chem.
SOC.Dalton Trans. 1991, 789.
[14] P. L. Timms, Adv. Inorg. Chem. Radiochem. 14 (1972) 121.
[I51 L. M. Cirjak, R. E. Ginsburg, L. E Dahl, Inorg. Chem. 21 (1982) 940.
[16] The species here is the complex anion [Re,Cl,,]3e[3].
[17] H. Brunner, N. Janietz, W. Meier, J. Wachter, E. Herdtweck, W. A. Herrmann, 0. Serhadli, M. L. Ziegler, J. Organonre!. Chem. 347 (1988) 237; H.
Brunner, N. Janietz, J. Wachter, B. Nuber, M. L. Ziegler, ibid. 367(1989)
197.
Novel Building Blocks for the Synthesis of Organic Metals
By Volker Enkelmann *
Organic crystals with special electrical, optical or magnetic
properties are attracting increasing interest as "unconventional materials".['] The special properties alluded to are
not determined alone by the electronic structure of the individual molecule, but are only manifested upon interaction of
many molecules in the solid state. It therefore does not suffice just to master the synthesis of suitable building blocks,
an important step is obtaining the desired interactions in
certain crystal structures. This has led to interest being always concentrated on a few model systems in which all steps,
from the synthesis of the starting components up to and
including reproducible crystallization, have been perfected.
High electrical conductivities in organic metals are, independently of the nature of the building blocks, always
associated with certain structural principles. The crystal
structure of the long known charge transfer (CT)-salt
TTFiTCNQ (TTF = tetrathiafulvalene, TCNQ = tetracyanoquinodimethane, Fig. I)['] exhibits the most important
[*I
features:L31 1) crystallization of donor and acceptor molecules in segregated stacks with 2) uniform interstack distances; 3) formation of mixed valence states, i. e. only partial
charge transfer between the stacks of the redox partners. The
charge transport takes place along the stacks, whereby in
TFFiTCNQ the two stacks contribute to the total conductivity independently of each other.
$
NC
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NC
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Priv.-Doz. Dr. V. Enkelmann
Max Planck-Institut fur Polymerforschung
Ackermannweg 10, W-6500 Mainz (FRG)
Angew. Chem. Inr. Ed. Engl. 30 (1991) No. 9
TCNQ
Verlagsgesellschaji mbH, W-6940 Weinheim. 1991
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N,
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DCNQI
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DCNAB
TTF
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Fig. 1. Section of the crystal structure ofTTF/TCNQ along the stack direction.
One could imagine that, by simple combination of many
different electron donors and acceptors, a sort of buildingblock kit would be available with which one could construct
almost any desired organic metal having tailor-made properties. Unfortunately this expectation is deceptive. In CT complexes of the type TTF/TCNQ, the redox potentials of the
two partners must be carefully harmonized so as to guarantee the required partial charge exchange. In addition, there is
the problem that the desired structures are not obtained
during the crystallization step following the redox reaction.
In many cases mixed stacks are formed in which donor and
acceptor molecules interchange. These difficulties can be
partly overcome if one of the two redox partners is replaced
by stable counterions with formation of radical ion salts.
However, crystal engineering, i.e. the selective growth of
crystals with predetermined structure, is still an unachieved
ideal.
A further reason for the number of building blocks being
limited lies in the interdisciplinary nature of the work, which
is necessary in this field. An overview of the organic metals
so far investigated in detail-TTF/TCNQ, radical cation
salts of TTF and analogously built fulvalenes such as, for
example, tetramethyltetraselenafulvalene (TMTSF) or bis(ethy1enedithio)tetrathiafulvalene(BEDT-TTF), radical anion salts of TCNQ, aromatic radical cation salts~']-shows
that these systems have all been investigated and optimized
over a long period of time by several working groups who
have contributed their expertise and knowledge from the
most diverse areas of organic synthesis, crystal growth
through to special methods of solid-state spectroscopy and
theory.
Whereas TTF has often been modified chemically (substitution, expansion of the IT system, replacement of S by Se or
Te), modification of the acceptor has so far been neglected,
so that TCNQ is still used almost exclusively. Hunig and
co-workers, however, have developed some valuable new
acceptors, which can give new impetus in the field of organic
one-step synthesis.t41 Like TCNQ they are two-step, reversible redox systems of the Wurster type which form stable
radical anions. On the basis of the known analogy between
the =C(CN), and =NCN groups it was expected that
DCNQI would behave analogously to TCNQ. This has indeed been confirmed, since CT complexes such as, e.g., TTF/
DCNQI can be obtained which behave similarly to the corresponding TCNQ salts. The special significance of DCNQI
compounds lies in their ability to form metal salts having
unusual proper tie^.[^-^] Whereas the TCNQ salts are
semiconductors, some DCNQI salts, e.g. [(2,5-dimethylDCNQI),Cu] , exhibit typical metallic behavior. Figure 2
shows the temperature dependence of the specific conductivity and the crystal structure of this salt. The specific conductivity, which increases steadily to values of 500000 S cm-'
with decreasing temperature, underscores the metallic character of this salt, which counts among the most conductive
organic metals (for comparison, the specific conductivity of
copper at room temperature is 650000 S cm-').
Remarkable, and apparently essential for the resulting extraordinary properties, is the tight tetrahedral coordination
of the copper ions by the C=N groups of four neighboring
P
d
'1
For some time now, N,N-dicyanoquinodiimine (DCNQI)
and its derivatives have been readily accessible by a facile,
1 1 22 0 VCH
Verlagsgesellschafl mbH. W-6940 Wemheim. 1991
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Fig. 2. Section of the crystal structure(top) and temperaturedependence of the
specific conductivity (I of [(2,5-DM-DCNQI),Cu] (bottom)[6].
0570-0833~9f~0909-1f22
$3.50+ 2510
Angew Chem hi.Ed Engl 30 ( 1 9 9 f ) No. 9
stacks. This coupling of the DCNQI stacks increases the
dimensionality and impairs Peierls transitions, which usually
lead to semiconducting low-temperature phases in one-dimensional systems. Similar couplings by sulfur-sulfur interactions between one-dimensional molecular stacks are observed in many salts of TTF derivatives which form superconducting low-temperature phases. The particular importance of the DCNQI salts lies in the fact that the strength of
the coupling can be varied by suitable choice of the counterion. When Cu is replaced by other metals isomorphic structures are obtained in which the C=N.-M distance increases
in the sequence Ag@,TI@,Lie. The conductivity decreases
with decreasing coupling strength, and the transition temperature to superconducting low-temperature phases increases in the same sequence. The metal salts of DCNQI thus
constitute ideal building blocks with which one can, within
certain limits, selectively change macroscopic properties
such as the conductivity, by suitable choice of molecular
parameters (counterion, substituition).
Recently Hiinig and co-workers have even gone one step
further. TCNQ and DCNQI are acceptors of the Wurster
type, whereas by means of a simple method for the synthesis
of dicyanoazobenzenes (DCNABs) we now have access to
acceptors of the inverse Wurster type (aromatic ground state
and quinonoid oxidized form). Preliminary results obtained
with CT complexes and salts of this new class of acceptors
have shown that they have redox potentials similar to TCNQ
and DCNQI and thus form radical anions of comparable
stability. Moreover, they do not differ substantially from the
well-known acceptors in their ability to form metal salts
and conductive CT complexes.['0*''I As an example, the
crystal structure of 2,5-dimethyl-DCNAB/TFF is shown in
Figure 3.
How are we to assess this new development? Since the
concepts underlying the makeup of an organic metal have
long been known, one cannot foresee any fundamentally
new insights being gained in this area. It must be remembered, however, that the structural principles outlined here
have been derived empirically from the common characteris-
tics of many organic metals and are therefore of rather a
qualitative nature. The new acceptors now readily accessible
through the works of Hiinig et al. improve the available set
of building blocks considerably, and, within certain limits,
make possible the tine tuning of molecular interactions by
suitable choice of components. So there is hope that in the
near future one will be able to make virtually quantitative
predictions. The problems which could be solved with the
help of new model structures include, in particular, the quantitative description of such macroscopic properties as the
temperature dependence of specific conductivity and phase
behavior (transition to semiconducting or superconducting
low-temperature phases) on the basis of molecular interactions along the stack and between neighboring stacks of
molecules.
One of the fundamental problems, however, remains unsolvable: the prediction of the crystal structure of organic
molecules and the planned assembly of structures with predetermined properties is possible only within modest limits.
Even with a much greater number of possible building
blocks and a greater number of empirical structural rules it
will depend upon the luck and skill of the experimenter as to
whether a structure with the desired properties will be obtained.
In comparison to other properties which are only realized
by the interaction of many molecules in larger units, the
electrical conductivity in organic crystals is relatively well
understood. Investigations on these systems therefore constitute ideal excercises for ambitious aims which are summarized under the concept molecular electronics. Here, one
hopes that molecules which act as switches, energy transport
media (molecular wires), logic units or storage systems can
be used to construct larger functional units and actively participate in the processing, transmitting, and storage of information. The developments outlined here hint at the course
that will have to be taken in the case of more complicated
systems: 1) developing an improved building-block kit of
suitable molecules; 2) synthesis of the simplest possible, well
characterized models, e.g. single crystals; 3) empirical determination of structure-property relationships; 4) extension of
these model concepts to the construction of more complicated structures.
German version: Angew. Chem. 103 (1991) 1142
[l] For an overview see Synth. Met. 41-43 (1991).
L----Y
Fig. 3. Section of the crystal structure of 2,5-Me2-DCNAB/TTF[I11
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 9
0 VCH
121 T. E. Phillips, T, J. Kistenmacher, J. P. Ferraris, D. 0. Cowan, J. Chem.
SOC.Chem. Commun. 1973,471.
[3] A. F. Garito, A. J. Heeger, Acc. Chem. Res. 7 (1974) 232.
[4] A. Aumiiller, S . Hiinig, Angew. Chem. 96 (1984) 437; Angew. Chem. Int.
Ed. Engl. 23 (1984) 447; S . Hiinig, P. Erk, Adv. Mater. 3 (1991) 225.
[5] A. Aumiiller, E. Hadicke, S. Hunig, A. Schatzle, J. U. von Schiitz, Angew.
Chem. 96 (1984) 439; Angew. Chem. Int. Ed. Engl. 23 (1984) 449.
[6] A. Aumiiller, P. Erk, G. Klebe, S. Hiinig, J. U. von Schiitz. H.-P. Werner,
Angew. Chem. 98 (1986) 759; Angew. Chem. I n t . Ed. Engl. 25 (1986) 740.
[7] S. Hunig, A. Aumiiller, P. Erk, H. Meixner, J. U. von Schutz, H.-J. Gross,
U. Langohr, H.-P. Werner, H. C. Wolf, C. Burschka, G. Klebe. K. Peters,
H. G . von Schnering, Synth. Met. 27 (1988) B181.
[8] R. Kato, H. Kobayashi, A. Kobayashi, Synth. Met. 27 (1988) 8263.
[9] A. Kobayashi, T. Mori, H. Inokuchi, R. Kato, H . Kobayashi, Synth. Met.
27 (1988) B275.
[lo] S. Hunig, T. Metzenthin, Angew. Chem. 103(1991) 610; Angew. Chem. Int.
Ed. Engl. 30 (1991) 563.
[ll] H. Almen, T. Bauer, S. Hiinig, V. KupEik, U. Langohr, T. Metzenthin, K.
Meyer, H. Rieder, J. U. von Schiitz, E. Tillmanns, H. C. Wolf, Angew.
Chem. 103 (1991) 608; Angew. Chem. Int. Ed. Engl. 30 (1991) 561.
Verlagsgesellschafi mbH. W-6940 Weinheim, 1991
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